Fugitive dust is generally defined as dust particles having an equivalent particle size or diameter in the range of 10 microns or greater whereas respirable dust is generally defined as dust particles having an equivalent particle size or diameter in the range of 10 microns or less. Fugitive dust particles are generally visible whereas respirable dust particles are generally not visible but stay suspended in the air. Both fugitive and respirable dust particles not only present a potential health hazard to personnel in the area but, depending upon the particular type of material involved, large concentrations of fugitive dust emissions can likewise be highly explosive. In material handling operations, dust more frequently becomes airborne at a processing point or at a conveyor belt transfer point, particularly, when large volumes of dusty material are involved and a change in elevation occurs at the transfer point.
One of the primary mechanisms of fugitive dust generation in bulk material handling operations is by dispersion of the dust in turbulent air induced to flow with falling or projected material streams. A stream of free-falling, or even sliding, bulk material induces in its wake a substantial flow of air as a result of frictional drag that the individual particles of the stream impose upon the surrounding air. At the end of its fall, when the material impacts, for example, a conveyor belt, a great deal of dust is generated at the impact location and the air becomes saturated with this dust. This dust laden air is then forced to escape under pressure. This scenario typically describes the primary method by which dust is generated in a typical conveyor-to-conveyor transfer operation.
Induced air flow is derived from the frictional drag imposed by air on the falling or projected material particles. While the particulate stream of material is falling through the air, frictional drag is tending to accelerate the surrounding air to the velocity of the falling stream. In other words, power or energy is being transferred from the falling solid material to the surrounding air. However, only a portion of this potential energy or power developed by the falling solid material stream is available to produce net air movement in the direction of the fall. The remaining balance of this potential energy goes to creating air turbulence. Also, many of the material particles are not sufficiently separated or spaced from adjacent particles to be subject to frictional drag, that is, some particles fall in the wake of other particles, particularly, in a compact stream that falls only a short distance and does not touch the associated chute or other containment walls. As a result, this induced air will flow from ingress around the discharging conveyor belt system to the belt loading zone of the receiving conveyor belt system and, if not allowed to be exhausted to some type of collection system, this induced air flow will pressurize the loading zone containment area causing entrained dust to blow out of leakage areas in the enclosure. This is true because, in a conventionally designed dust control transfer containment system commonly employed in the industry, the induced air flow typically has no other flow path in which to dissipate.
Still further, in the known material handling transfer systems, the material being transferred from a first elevated conveyor belt system to a second lower conveyor belt system is typically allowed to free-fall and directly impact the second conveyor belt system. This direct impact of the falling material stream onto the conveyor belt system provided in the loading zone of the transfer point typically produces a "trampoline" or pumping action of the conveyor belt due to the impact of the falling material, which pumping action provides another mechanism for fugitive dust generation in that it pumps more dust and particulate back into the enclosure system as well as out through the seal skirting surrounding such containment system. Where the material transfer is totally enclosed, such as is typically the case in a conveyor-to-conveyor transfer system, the induced air component usually predominates and must be exhausted in some fashion in order to control dust emissions.
Using the coal industry as an example, there are several methods commonly employed for controlling coal dust that becomes airborne in coal handling and processing facilities. These dust control systems must accomplish two primary goals, namely, (1) maintain a non-hazardous work environment for both personnel and equipment, and (2) minimize the emissions of dust into the ambient atmosphere. These goals must be met to the satisfaction of the operator as well as to the satisfaction of both state and federal regulatory agencies which deal with environmental and occupational health issues. Such known dust control systems typically include suppression type systems such as water spray systems as well as foam and water misting systems, dust filter systems, enclosure systems, and a baghouse and fan system to name a few.
All of the known dust control systems commonly employed for controlling fugitive dust emissions suffer from certain limitations and disadvantages depending upon the particular material application involved. For example, known water spray systems typically depend upon the application of a large quantity of water that is sometimes mixed with a wetting or binding agent. The objective of this system is to coat the surface of the coal or other material product so that finer coal particles adhere to the larger particles to prevent them from becoming airborne. The use of large quantities of water is deleterious to product quality in that it increases the moisture content and decreases the as-shipped calorific value. Furthermore, in a coal mining operation, water should not be applied down stream of the coal-quality sampling point, since the sample would not be representative of the product being shipped. Also, importantly, these types of dust control systems are often subject to freezing and becoming disabled when operating in freezing temperatures since water forms ice coatings on the equipment such as the conveyor belts, idlers, drive pulleys and other components resulting in production interruptions and damage.
Known foam and water misting systems generally use much less water as compared to water spray systems, thus avoiding significant impact to the product quality. These systems are typically designed to attack the dust directly. The agent is applied within an enclosure where the dust is contacted, coated and agglomerated together causing such particles to fall out of the air back onto the belt conveyor system. Experience has shown that in high capacity coal handling systems, mists and foams do not sufficiently coat and contact the dust particles so as to result in an efficient dust removal system. In freezing temperatures, these systems also tend to freeze and become disabled. Even small amounts of water in extreme cold are hazardous to many different types of material handling equipment, particularly coal handling equipment.
Known dust filter systems typically use a fan and filter housing directly attached to the hood structure that surrounds a processing or transfer point to trap and immediately return dust to the product stream. The control theory employed in these systems is similar to that used in baghouse systems, except that the dust is not transported to a remotely located baghouse filter system. Dust filter systems are inexpensive but are only effective for very small material handling capacities.
Still further, known enclosure systems simply keep the airborne dust from escaping into the ambient environment thereby allowing the dust to accumulate inside the enclosure. Such systems can be used only where personnel and hazardous equipment are not present. Examples of such areas in a coal handling system include storage silos, storage barns and stilling sheds over truck dumping stations.
The known dust control and collection system most frequently used on large scale coal handling systems is a baghouse and fan system. This system actively ventilates induced air flow from a material handling transfer point and transports such dust to a remotely located baghouse filter system where such dust is filtered back to the coal stream. This type of system is expensive to operate and is particularly prone to malfunction, especially in freezing weather due to the high moisture content of the subbitumious coal. Baghouse systems would commonly be specified for most process and transfer points where a structural enclosure is not acceptable.
It is therefore desirable to provide a material handling dust control transfer system which effectively exhausts the induced air component associated with a bulk material handling operation and which effectively controls fugitive dust emissions during such transfer operations. It is also desirable to provide a system which requires minimum maintenance, no power consumption, no moving parts, no water, mist or other suppression agents, and which is relatively inexpensive to install and operate as compared to other known dust emission control systems. Accordingly, the present invention is directed to overcoming one or more of the problems as set forth above.